1. Field of the Invention
The present invention relates to an optical element, and an optical system and an optical apparatus using the same.
2. Description of the Related Art
Conventionally, an anti-reflection film for increasing the amount of transmitted light and avoiding the occurrence of ghost and flare caused by unwanted light has been coated on one side or both sides of the optical element employed in the imaging optical system provided in a photographing lens for use in an optical apparatus such as a video camera, a photographic camera, a television camera, or the like. In particular, for adaptation to high resolution such as high-definition (HD) of recent video cameras and television cameras and high image quality of recent photographic cameras, there is also a growing need for higher anti-reflection performance. Here, in order to obtain high anti-reflection performance, a material which is used for the outermost layer of an anti-reflection film needs to be a material having a low refractive index. Examples of such material having a low refractive index include inorganic materials such as silica, magnesium fluoride, or the like or organic materials such as silicone resin, amorphous fluorine resin, or the like. Furthermore, there is also a technology for lowering a refractive index by forming gaps within a layer made of silica or magnesium fluoride. For example, when gaps (30% of total volume) are provided within the thin film layer made of magnesium fluoride having a refractive index of 1.38, the refractive index can be lowered to 1.27. As a method for forming gaps within such a layer, Japanese Patent Laid-Open No. 2006-151800 discloses a method for manufacturing an anti-reflection film (silica aerogel film) by using a low refractive index material which is formed of silica fine particles or magnesium fluoride fine particles together with a binder with gaps being formed between fine particles. In contrast, Japanese Patent Laid-Open No. 2004-258267 discloses an anti-reflection film having a low refractive index layer which is formed of hollow fine particles, each of which has a vacancy therein, held together by a binder with binding between fine particles being enhanced by filling a space between hollow fine particles with another binder, and a method for manufacturing the same.
Here, in the anti-reflection film disclosed in Japanese Patent Laid-Open No. 2004-258267, hollow fine particles are highly filled in a low refractive index layer and binding between hollow fine particles is enhanced by filling gaps between hollow fine particles with a binder so as to ensure a low refractive index with the aid of a vacancy provided inside each of the hollow fine particles. However, a highly-filled binder may cause an undesirable increase in refractive index.